I have never before seen something yucky on screen make an entire cinema audience suddenly hunch forward and bury their heads in their laps at the same time, as if in some secular mosque for wimps. But that's what we all did during Danny Boyle's new movie, 127 Hours, the gruelling true-life story of Aron Ralston, a twentysomething guy who in 2003 went hiking and climbing in the breathtakingly beautiful Blue John Canyon in Utah and got his arm immovably trapped under a boulder. After days of frantic screaming, futile tugging and fruitless shoving, with food and water close to zero, Ralston looked at the knife he'd brought with him and began to weigh up what our politicians call the "hard choices" about what he now had to leave behind.

James Franco plays Ralston, on screen in closeup for pretty much the entire time, and his face nicely conveys his transition from smug extreme-sports badass to very scared little boy, contemplating something that would terrify any adult. I couldn't help remembering Marlon Brando's gibbering, sub-Nietzschean epiphany in Apocalypse Now; after having once inoculated the children of some remote jungle tribe against polio, he later returned to find the disapproving elders had chopped off all the injected arms. ("My God! The genius of that! The will to do that!")

Ralston's sport is not, in fact, precisely hiking or climbing but something in between: "canyoneering", of which I confess I hadn't heard before this. The canyoneer appears to brace himself in the rock crevice and go into a semi-controlled free-fall slither. This, on its own, was quite scary enough for me. On a dangerous high from a romantically charged encounter with two attractive female hikers, and with a promise of a party later, Ralston swaggers off solo, brimming with hubris and has his terrible accident with the boulder. The implacable mass of trillion-year-old rock meets quivering flesh: the vast, unimaginable forces of geology and geography engulf this tiny, vulnerable life.

My reservation about the film is that I found myself wanting to see a factual movie about Aron Ralston — something like Touching the Void, Kevin Macdonald's classic 2003 talking-head documentary about luckless climbers in the Peruvian Andes. A film like this would have got closer to what was going on in Ralston's head and heart. And scenes showing Ralston creating his Blair-Witch-type video diary inevitably made me long to see the real thing, but Aron Ralston himself, understandably, refuses to let this intensely painful footage be shown.

Well, there's no doubt about the skill and flair with which Boyle re-imagines Ralston's awful jam, candidly but not too explicitly showing how he got out of it; Franco's performance is winning yet unsentimental and the landscape images conjured by cinematographers Anthony Dod Mantle and Enrique Chediak are tremendous.